This invention relates to a multi-element antenna and in particular, but not exclusively, to a multi-element antenna and associated switching arrangement designed for use in a monopulse radar collision avoidance system.
The Traffic alert and Collision Avoidance System (TCAS) is an implementation of the Airborne Collision Avoidance System which is fitted to all aircraft over a certain weight and/or passenger carrying capacity, as mandated by the International Civil Aviation Organisation. A recent implementation of TCAS employs an eight element circular antenna array which is fed through a conventional Butler matrix to generate circular phase modes in the array. The circular modes are phase shifted and combined in a sum/difference hybrid to provide a monopulse radar system with claimed resolution of 2° or better. However, TCAS relies upon the relative phase of two circular modes to detect a potential hazard and as such is sensitive, in certain mounting arrangements, to multipath reflections from parts of a host airframe leading to reduced resolution and potential false alarms.